Arrangement including a substrate for power components and a heat sink, and a method for manufacturing the arrangement

ABSTRACT

In an arrangement composed of a substrate and a heat sink, the substrate is provided on a first side with at least one power component arranged on a first large-surface printed circuit trace and, on a second side opposite the power component, with a second large-surface printed circuit trace, which is connected in a thermally conductive manner to the first printed circuit trace via via holes. The substrate is mounted at the second side onto the heat sink in a thermally conductive manner, in order to achieve a good heat coupling of the substrate to the heat sink and the same time to avoid an undesirable electrical contact between the potential-carrying printed circuit traces and the heat sink. The substrate having spacer elements arranged on the second side is placed onto the heat sink and to hold it at a defined distance from the heat sink, the gap formed by the distance between the substrate and the heat sink being filled with a thermally conductive filler.

FIELD OF THE INVENTION BACKGROUND INFORMATION

A conventional arrangement is described in German Patent Application No.1 95 28 62. In this publication, as the substrate a circuit board isindicated, which on its upper side is provided with an electroniccircuit, which includes at least one power component that generateswaste heat. Beneath the power component, the circuit board is providedwith via holes, which divert the heat generated by the power componentto the lower side of the circuit board. Between lower side of thecircuit board and a control unit housing functioning as a heat sink, athermally conductive filler is arranged. During operation, the heatproduced by the power components is diverted via the via holes to thelower side of the circuit board and from there is delivered via thethermally conductive filler to the housing, which functions as a heatsink. In this context, it is disadvantageous that printed circuit tracesthat conduct potential on the lower side of the circuit board can comeinto contact with the heat sink during assembly of the circuit board inthe control unit. A short-circuit that is caused in this way can damageor destroy the sensitive electronic components on the circuit board.

In addition, German Patent Application No. 1 97 23 409 describes anarrangement having a substrate and a heat sink. On the upper side of thecircuit board, a power component is deposited on a large-surface printedcircuit trace, which is connected by via holes to a large-surfaceprinted circuit trace on the lower side of the circuit board. On thelower side of the circuit board, a metal layer is deposited under thelarge-surface printed circuit trace arranged there and over aninsulation layer, which in turn is deposited through a solder stop maskonto a control unit housing part that is provided as a heat sink. Inthis arrangement, although an electrical contact between the printedcircuit traces and a heat sink is prevented by the insulation layer, itis disadvantageous that the insulation layer and the further metal layercomplicate the direct heat transfer to the heat sink, increase the spacerequirements of the arrangement, and also make manufacturing moreexpensive.

SUMMARY OF THE INVENTION

The arrangement according to the present invention avoids thedisadvantages arising in the conventional arrangements. As a result ofspacer elements placed on the side of the substrate opposite the powercomponents and a thermally conductive filler introduced between thesubstrate and the heat sink, an effective heat coupling of the substrateto the heat sink is advantageously achieved, on the one hand, and anundesirable electrical contact between the printed circuit tracescarrying potential and located on the side of this substrate and theheat sink is dependably avoided, on the other hand. In addition, aparticularly space-saving arrangement can be realized. Additionallayers, making manufacturing more expensive, such as an additionalinsulating layer or a further metal layer deposited onto the insulationlayer, are not necessary, so that the costs in this regard can be saved.

It is also advantageous if the spacer elements are composed of conductorsurface elements on the lower side of the substrate, which are coatedusing a preselected quantity of solder, since for this purpose, inparticular for substrates that are fitted with components on both sides,no additional manufacturing step is required. The conductor surfaceelements can be manufactured together with the connecting surfaces ofthe electronic components provided on the lower side, and can be coatedwith solder.

A solder resist deposited on the side of the substrate opposite thepower components prevents solder from mistakenly reaching locations thatare not provided therefore during the application of the solder.

If the power component and the heat sink have the same electricpotential, it is advantageous to integratedly manufacture the conductorsurface elements directly in the second large-surface printed circuittrace, since in this way the heat transfer is improved.

In addition, it is advantageous if the thermally conductive fillerprovided between the substrate and the heat sink is a thermallyconductive adhesive or a thermally conductive adhesive foil, by whichthe substrate can also be mechanically secured to the heat sink.

The spacer elements resting on the heat sink can advantageously also beused as a ground connection of the substrate to the heat sink and forimproving the EMV behavior (electromagnetic compatibility).

In addition, the present invention relates to a method for manufacturingan arrangement composed of a substrate and a heat sink. In particular,in the case of substrates having components on both sides, no additionalmanufacturing steps are necessary for carrying out the method accordingto the present invention. The conductor surface elements can bemanufactured together with the printed circuit traces provided on thesecond side. The deposition of solder necessary for the manufacture ofthe spacer elements can be carried out together with the soldering ofthe connecting surfaces for components, which makes the methodparticularly economical, since scarcely any additional costs arise formanufacturing the spacer elements.

It is advantageous to imprint the solder onto the conductor surfaceelements in a solder paste imprinting station, since this technology isparticularly well suited for depositing a defined quantity of solder andcan be controlled very well. In a subsequent reflow soldering step, thesolder is melted, the spacer elements being formed at a height definedby the quantity of solder deposited. The reflow solder step canadvantageously take place with the reflow soldering of the SMDcomponents provided on the substrate.

It is easy to apply a thermally conductive adhesive or a thermallyconductive adhesive foil first onto the heat sink and subsequently toplace the substrate onto the heat sink coated with the adhesive or theadhesive foil such that the spacer elements are impressed into theadhesive, they being able to contact the heat sink via the solder layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross-section of a conventional arrangement having aninsulation layer and an additional copper plate.

FIG. 2 shows a cross-section of a first exemplary embodiment of anarrangement according to the present invention.

FIG. 3 shows a cross-section of a second exemplary embodiment of thearrangement according to the present invention.

DETAILED DESCRIPTION

FIG. 1 shows a conventional arrangement for diverting the waste heatproduced in a control unit by a power component. On upward-facing firstside 8, a circuit board 2 is provided with a first large-surface printedcircuit trace 10 and, on the downward-facing second side 9, with asecond large-surface printed circuit trace 11. The first large-surfaceprinted circuit trace 10 and a second large-surface printed circuittrace 11 are well connected with each other in a thermally conductivemanner, by numerous via holes 4 extending across the circuit board. Apower component 3, for example, an SMD component, is applied onto firstlarge-surface printed circuit trace 10, the component being conductivelyconnected via connections 14 to further printed circuit traces 12,insulated from large-surface printed circuit trace 10, on upper side 8of circuit board 2. For simplicity's sake, only one printed circuittrace 12 is depicted. On lower side 9, further potential-carryingprinted circuit traces 13 are located, which are arranged so as to beinsulated from second large-surface printed circuit trace 11. Inaddition, on lower side 9, provision is made for further undepicted SMDcomponents, which are soldered on the lower side to undepictedconnecting surfaces. Furthermore, a copper plate 6, which is used as aheat sink, is deposited onto second large-surface printed circuit trace11 and printed circuit traces 13 via an electrically insulating,thermally conductive insulation layer 5. A solder resist 15, in turn, isapplied onto copper plate 6, the resist preventing a deposition of thesolder on the copper during the soldering of the connecting surfaces forthe SMD components. Underneath solder resist 15, provision is made for athermally conductive adhesive 7, which is applied onto a base part 1control unit housing, the base part being provided as a heat sink. It isalso known to press the copper plate directly onto the heat sink using ascrew-type mounting means. In any case, the heat produced by powercomponent 3 is dissipated via the via holes onto large-surface printedcircuit trace 11 and via insulation layer 5 onto copper plate 6. Fromthere, the heat is transferred to heat sink 1 by solder resist 15 eitherdirectly or via thermally conductive adhesive 7. If circuit board 2 wereapplied to the heat sink without insulation layer 5 and copper sink 6, ashort-circuit between potential-carrying printed circuit traces 13 couldlead to damaging the components. However, in the arrangement shown inFIG. 1, it is disadvantageous, that in order to apply insulation layer 5and additional copper plate 6, two further, separate manufacturing stepsare required.

FIG. 2 shows a first exemplary embodiment of an arrangement according tothe present invention for dissipating the waste heat of a powercomponent, the arrangement being preferably installed in an electroniccontrol unit of a motor vehicle. At least one power component, e.g., apower semiconductor, is applied onto a large-surface printed circuittrace 10 on upper side 8 of a substrate 2, which can be a circuit board,a hybrid, or another substrate furnished with an electronic circuit. Inthe exemplary embodiment depicted here, substrate 2 is a circuit boardhaving components on both sides. As can be seen further in FIG. 2, aconnection 14 of power component 3 is electrically connected to aconnecting printed circuit trace 12 of upper side 8 of circuit board 2,the connecting printed circuit trace being insulated from printedcircuit trace 10. On lower side 9 of circuit board 2, a secondlarge-surface printed circuit trace 11 is applied, which is connected ina thermally conductive manner to first printed circuit trace 10 on upperside 8 by via holes 4. In addition, still other undepicted printedcircuit traces, associated with the circuit, as well as a number ofconnecting surfaces for SMD components are provided on lower side 9. Asis depicted in FIG. 2, on lower side 9 of circuit board 2, provision isfurther made for conductor surface elements 17, which can bemanufactured together with the remaining printed circuit traces andconnection surfaces and can be made from the same material on the lowerside of the circuit board. This can take place using the customary knowntechnologies. In the exemplary embodiment depicted in FIG. 2, a voltageis transmitted to second large-surface printed circuit trace 11 frompower component 3 by via holes 4. Conductor surface elements 17 aretherefore arranged on the substrate insulated from printed circuit trace11. Furthermore, a solder resist 15 is applied in the known manner ontolower side 9, provision being made for recesses in the solder resist atthe locations of conductor surface elements 17 and at the undepictedconnection surfaces for SMD components. In the manufacture of thearrangement depicted in FIG. 2, circuit board 2 is turned with lowerside 9 facing upwards, and, in a solder paste imprinting station, isimprinted using solder paste. In this context, solder is applied ontoconductor surface elements 17 and onto connection surfaces for SMDcomponents. Solder resist 15 prevents solder from reaching other circuitparts in the process. After the imprinting of the solder, circuit board2 is conveyed to an assembly machine, which impresses the SMD componentsinto the solder paste applied on the connection surfaces onupwards-facing lower side 9 of the circuit board. Subsequently, thecircuit board passes through a reflow solder station, in which thesolder is melted. In this context, it is advantageous if the SMDcomponents are soldered to the connecting surfaces and, at the sametime, the spacer elements 17, 18 are formed. This takes place byliquefying solder 18 that is impressed onto conductor surface elements17, in the reflow oven, and, as a result of the surface tension of thesolder on surface pieces 17, solder humps or solder caps of a definedsize are formed, whose shape is a function only of the size of theconductor surface elements 17 and of the quantity of solder that isimpressed. In particular, using the described method, it is possible toconfigure all the spacer elements having one precisely defined uniformheight. In this context, it is advantageous that the solder pasteimprinting step and the reflow soldering step must in any case becarried out for circuit boards having components on two sides, so thatno additional manufacturing step is required for producing the spacerelements. In place of the manufacturing process described above, it isalso possible to moisten the conductor surface elements, e.g., in a wavesolder bath using liquefied solder. It is advantageous that the spacerelements be manufactured at a defined height. After the manufacture ofspacer elements 17, 18, a thermally conductive adhesive 7 is appliedonto a heat sink 1 using a dispensing device. In a further exemplaryembodiment, provision is made for employing a thermally conductive foil,having adhesive on both sides, in place of the adhesive. Functioning asheat sink is a housing part of the control unit, for example, thehousing base. Circuit board 2 is then placed onto the adhesive with itslower side 9 turned toward heat sink 1 and, in this manner, is pressedin the direction of the heat sink so that spacer elements 17, 18penetrate into adhesive 7. In this context, they can contact heat sink 1and thus obtain a minimal spacing. In this context, a trough-shapedrecess, not depicted in FIG. 2, in the housing base of the control unitreceives the components arranged on the lower side of the circuit board.As a result of spacer elements 17, 18, a defined gap is formed betweenthe lower side of the circuit board and the heat sink, the gap, as shownin FIG. 2, being completely filled with adhesive 7. Because the spacerelements can be manufactured having a defined small height, the gap canbe set very small without the heat sink contacting the lower side of thecircuit board, which improves the heat removal to the heat sink.

The arrangement shown in FIG. 2 can, also be manufactured, for example,in that circuit board 2 having spacer elements 17, 18 is first placedonto the heat sink, only subsequently an adhesive capable of capillaryflow being introduced into the gap between the circuit board lower sideand the heat sink.

Spacer elements 17, 18 can advantageously be used also for the EMVprotection (electromagnetic compatibility) of the arrangement. Since thespacer elements are composed of an electrically conductive material, anelectrical contact to the heat sink is generated by them, i.e., spacerelements and heat sink have the same potential. If at least someconductor surface elements 17 are connected to printed circuit tracesassociated with the circuit, then, via the spacer elements, a short andthus low-radiation ground connection can be realized.

A second exemplary embodiment is depicted in FIG. 3. The same numbersdenotes the same parts. The arrangement depicted in FIG. 3 differs fromthe arrangement depicted in FIG. 2 in that power component 3 and heatsink have the same potential. Therefore, the conductor surface elements17 can advantageously be directly integrated into second large-surfaceprinted circuit trace 11 of substrate 2. Solder resist mask 15, appliedon second side 9 of substrate 2, has recesses which define conductorsurface elements 17. Solder caps 18, as described above, are formed onthese conductor surface elements. Subsequently, the substrate is placedonto heat sink 1 or adhesive 7. Since power component 3, printed circuittrace 11, and heat sink 1 have the same potential, no short-circuit isgenerated by spacer elements 18. In comparison to the first embodimentshown in FIG. 2, the heat transfer by the arrangement of solder caps 18on second large-surface printed circuit trace 11 can be improved.

What is claimed is:
 1. An arrangement comprising: a substrate; a heatsink; at least one power component situated on a first side of thesubstrate; a first large-surface printed circuit trace, the at least onepower component being situated on the first large-surface printedcircuit trace; a second large-surface printed circuit trace situated ona second side of the substrate the second side being opposite to the atleast one power component, the first large-surface printed circuit tracebeing coupled to the second large-surface printed circuit trace via atleast one hole in a thermally conductive manner; and a plurality ofspacer elements located between the second side of the substrate and afirst surface of the heat sink, wherein: the substrate is maintained ata predetermined distance from the heat sink by the plurality of spacerelements so that a fixed gap is formed between the second side of thesubstrate and the first surface of the heat sink, the fixed gap beingfilled with a thermally conductive filler; and the plurality of spacerelements is formed by a plurality of conductor surface elements, each ofthe conductor surface elements being coated with a predetermined amountof solder.
 2. The arrangement according to claim 1, wherein contactsbetween the conductor surface elements are coated with solder, andwherein the contacts enable the plurality of spacer elements toelectrically couple the heat sink to the substrate so that the heat sinkprovides a ground connection for the substrate.
 3. The arrangementaccording to claim 1, further comprising: a solder resist coating thesecond side of the substrate without coating the conductor surfaceelements.
 4. The arrangement according to claim 3, wherein the at leastone power component and the heat sink have substantially the sameelectrical potential, and wherein the conductor surface elements areintegrated with the second large-surface printed circuit trace.
 5. Thearrangement according to claim 1, wherein the thermally conductivefiller is one of a hardenable thermally conductive adhesive and athermally conductive foil which has an adhesive on both sides thereof.6. The arrangement according to claim 1, wherein the arrangement isutilized in an electronic control unit.
 7. A method of manufacturing anarrangement including a substrate, a heat sink, at least one powercomponent situated on a first side of the substrate, a firstlarge-surface printed circuit trace accommodating the at least one powercomponent thereon, and a second large-surface printed circuit tracesituated on a second side of the substrate, the second side beingopposite the first side, the first large-surface printed circuit tracebeing thermally conductively coupled to the second large-surface printedcircuit trace via at least one hole, the method comprising the steps of:providing a plurality of conductor surface elements on the second sideof the substrate; applying a solder resist to the second side of thesubstrate without applying the solder resist to the conductor surfaceelements; coating the conductor surface elements with a predeterminedamount of a solder, the predetermined amount of the solder beingselected to form a plurality of spacer elements of a preselected height;placing the substrate, and the spacer elements situated thereon, on afirst surface of the heat sink; and introducing a thermally conductivefiller between the second side of the substrate and the first surface ofthe heat sink.
 8. The method according to claim 7, further comprisingthe step of: imprinting the solder onto the conductor surface elementsat a solder paste imprinting station, wherein the spacer elements aregenerated in a reflow solder station by performing a subsequent reflowsoldering procedure on the substrate.
 9. The method according to claim7, further comprising the steps of: applying the thermally conductivefiller to the first surface of the heat sink, the thermally conductivefiller being one of a thermally conductive hardening adhesive and athermally conductive foil having an adhesive on both sides thereof; andpressing the substrate onto the first surface of the heat sink so thatthe spacer elements pass through the one of the thermally conductiveadhesive and the adhesive on the thermally conductive foil to contactthe first surface of the heat sink.